Explosions rocked Monrovia as rebels and government forces battled at key crossings leading to President Charles Taylor's downtown stronghold. Government commanders and residents reported that the rebels were stepping up their eight-day drive into the city.

Rebels are making their third attempt in two months to take Monrovia, an isolated and disease-ridden city of at least 1.3 million people.

The main rebel movement "needs to show that they have regard for the people of Liberia, that it is not indifferent to the great human suffering that is taking place here," Ambassador John W. Blaney told reporters at the heavily guarded U.S. Embassy.

The ambassador urged the rebel movement Liberians United for Reconciliation and Democracy to pull back to the natural boundary of the Po River, six miles outside the capital.

The withdrawal would open up the port and camps outside the refugee-choked city for food and aid.

Blaney said Taylor had agreed to the proposal.

The American ambassador asked the rebels to do the same.

"If they want to get to a post-Taylor era, this is the way to do it," he said.

Sekou Conneh, the rebels' civilian chairman, told the Associated Press that they would retreat only when peacekeepers were in place.

"We agree to fall back, but we want the peacekeepers to come," Conneh said. "We don't want to hand over the port to Charles Taylor."

Under international pressure to intervene, President Bush has ordered U.S. ships to take up positions off the coast of Liberia, ready to support a West African-led peace force, which has been delayed amid debates over funding.

New bombardments crashed into Monrovia neighborhoods on Sunday.

One round landed before dawn on a tin-roofed shack near the Old Bridge, killing four, according to aid workers collecting bodies in the area. Another fell on a nearby house, killing two.

A shell that had fallen nearby late Saturday killed an entire family -- eight adults and two children, humanitarian aid workers said.